Fix GCC's -Wunterminated-string-initialization warning in edns0.c.

GCC complains that writing the five-character "ODNS\0" string into
a four-element char magic[4] array truncates the NUL character.
The warning's rationale is that this is incompatible with C++, or
maybe non-intentional.

GCC 8 has added a nonstring variable attribute, clang 20.1 does
not yet support this, but clang's Git head does.

Add an ATTRIBUTE_NONSTRING macro, currently only defined on GCC >= 8
as __attribute__ ((nonstring)).  This successfully suppresses
the warning on Fedora Linux 42's default compiler.

The alternative would be to replace the "ODNS" literal by {0} and
instead memcpy(opt.magic, "ODNS", sizeof(opt.magic)); on the next line,
which is correct, C++ compatible, but also less concise.

src/edns0.c

@@ -494,7 +494,7 @@ int check_source(struct dns_header *header, size_t plen, unsigned char *pseudohe
 #define UMBRELLA_DEVICESZ   sizeof(daemon->umbrella_device)
 
 struct umbrella_opt {
-  u8 magic[4];
+  u8 magic[4] ATTRIBUTE_NONSTRING;
   u8 version;
   u8 flags;
   /* We have 4 possible fields since we'll never send both IPv4 and